In critical care and other medical care situations, the parameter of fluid status is an important parameter to be monitored. The patient's fluid status generally refers to the volume, or change in volume, of blood presently in the circulatory system of the patient.
A change in the patient's blood volume due to hypervolemia or excessive fluid, or hypovolemia, or reduced fluid volume, can result in various autonomic responses by the organ systems of the patient. As baroreceptors detect a change in the fluid volume, the organ function similarly changes to accommodate for the addition or loss of fluid. These autonomic responses attempt to maintain appropriate bodily function in response to the change in fluid volume.
Another effect of a change in fluid volume is the impact on pharmacokinetic and pharmacodynamic models used by clinicians in determining the proper drug dosage, the resulting drug concentration in the body, and the body's metabolism of the drugs currently in its system.
One exemplary situation where the fluid status of the patient is an important physiological parameter is that of a critical care situation, such as during an invasive surgery. During such surgery, the anesthesiologist must continuously monitor the patient's vital signs to determine the proper drug concentrations to be delivered to the patient. The vital signs, including the patient's heart rate, temperature, blood pressure and breath rate, all may be affected by the patient's autonomic response to a change in fluid status. These vital signs are all important to determine the proper drug concentrations and fluids to be delivered to the patient. This is conducted in a situation in which the patient may experience significant changes in fluid status. Sources of fluid status change include bleeding, dehydration via tissue exposed to air at the surgical site, urine production, and the administration of intravenous fluids, including crystalloids or colloids.
Current techniques for monitoring the patient's fluid status are limited for being complex, inaccurate, or subjective. Techniques such as measuring capillary refill time (or the time that it takes for blood to return to nail bed tissue after pressure has been applied) are subjective and qualitative evaluations of fluid status. Techniques such as transesophagal echo that utilize ultrasound to monitor fluid movement in the thoracic cavity are invasive, especially in situations when the patient requires mechanical ventilation. Alternatively, an estimation of fluid status may be made by actively charting all of the fluid into and out of the patient. This estimation is labor intensive and is limited in its ability to provide timely fluid status levels.